STEP(5)

The importers cover advanced and faceted B-reps, B-reps with voids, manifold surface models, shell-based surface models, mapped items, solid replicas, geometric sets, and supported wireframe representations. Geometry normally becomes a BRL-CAD brep unless an exact native mapping is available. Units, representation uncertainty, and placements are applied during conversion.

Open boundaries in a shell-based surface model may be intended sheet geometry or construction content. They are imported by default. The step-g --skip-open-shells option filters only OPEN_SHELL boundaries from those surface models; closed surface boundaries and solid B-rep topology remain enabled.

Bounded geometric sets retain exact curves as B-reps and Cartesian points as pnts objects. Mixed sets become combinations. Product-owned constructive geometry planes become datum planes; placement axes remain coordinate frames rather than standalone geometry.

STEP and OpenNURBS use different spline-knot storage conventions. The importer validates the expanded STEP knot vector and removes only the two extra endpoint knots OpenNURBS does not store. Closed unclamped surfaces are normalized only when a dense before-and-after audit remains within the source uncertainty.

AP203e2, AP214, and AP242 share exact conversion for supported EXTRUDED_AREA_SOLID, REVOLVED_AREA_SOLID, EXTRUDED_FACE_SOLID, REVOLVED_FACE_SOLID, and SWEPT_DISK_SOLID entities. Area and face sweeps become exact B-reps. Profiles may include inner void loops.

A linear swept disk becomes a native cylinder or cylinder-difference region. A direct circular sweep becomes a full or partial toroidal B-rep, including an optional inner void. Other nonlinear directrices are reported rather than approximated. Trimmed and drafted face-sweep subtypes are not treated as their base sweeps because that would discard termination, offset, or draft semantics.

Triangulated faces, strips, fans, surface sets, shells, and solids use a direct BOT path. Coordinates, indices, degeneracy, normals, and winding are validated. Tessellated solid intent produces a volume BOT only when the triangles are closed and consistently oriented; incomplete input remains a diagnosed surface BOT. Tessellated point, curve, wire, and geometric sets do not yet have a direct mapping.

Stable values are stored as step: attributes on their owning products or occurrences. Product fields include original name and identifier, descriptions, revision, and definition identity. Presentation fields retain colour, transparency, style name, and layers. Source representation contexts record length and angle factors, normalized units, and uncertainty as provenance rather than automatic future export instructions.

Reference-linked graphs without a direct object equivalent are encoded in the portable, versioned _STEP_METADATA binary object. Locator attributes are written on both that object and _GLOBAL. For compatibility, a modest graph is also mirrored as directly editable _GLOBAL attributes; large graphs use only the locator there to avoid poorly scaling attribute operations. g-step reads the binary object first and merges compatible global attributes.

Source Part 21 identifiers are internal graph anchors only. The exporter allocates new instance numbers and remaps every supported reference after validating that its target still has one emitted counterpart.

AP203e2, AP214, and AP242 share colour, transparency, layer, material, and typed property round trips. A native BRL-CAD colour overrides retained STEP colour on export. Product properties and material properties remain distinct. Supported values include descriptive text, Cartesian coordinates, and typed numeric measures.

Numeric properties retain measure type, seven dimensional exponents, a normalized label, and a recursive unit graph. SI, context-dependent, conversion-based, and derived units can be re-authored, including conversion units nested in derived units. The graph takes precedence over a convenient label and is validated instead of guessed.

Constrained measure types must agree with their dimensions. Invalid pairs remain visible in the report but are not stored as exportable properties. Safe repair recognizes one narrowly defined producer defect: a positive density with exactly mass^1 and length^3 is normalized to mass^1 times length^-3 without changing its value. The repair requires the complete unit graph and is disabled by exact, no-repair, and strict import. No other dimension is inferred from a human-readable label.

AP203 edition 1 does not author material, direct product-property, or presentation requests. Such retained data is reported as unsupported rather than silently discarded.

The retained graph covers supported approvals, people, organizations, addresses, dates and times, documents, security classifications, product categories, alternate products, usage substitutes, and effectivities. Import preserves exact values and references; export reconstructs only records that are legal in the selected target schema.

AP203 uses narrower CC_DESIGN_…​ assignments, closed vocabularies, and a mandatory management closure. The exporter supplies documented deterministic neutral values only for required ordinary closure, never to claim that an unsupported source record was preserved. AP203 serial, dated, and lot effectivities require complex CONFIGURATION_EFFECTIVITY structures. Modern APs use broader APPLIED_…​ forms, but still differ: for example, AP214 restricts several effectivity assignments and AP203e2 has no product-category relationship entity.

Dependencies, cycles, ambiguous identities, or assignments that violate the target EXPRESS rules are omissions. Each retained node receives an emitted, unsupported, or malformed disposition and reason in the export report.

Occurrence metadata is guarded by the current child name on its parent combination. If an edited tree no longer matches, the values are omitted instead of migrating to another member.

EXPRESS defines legal data structures, but recommended practices establish the entity patterns exchange systems use. Relevant references include:

BRL-CAD treats an EXPRESS long form as generated-code input, not a generic run-time data file. Building every schema in the STEP module library would add substantial generation, compilation, link, and installation cost without providing application-protocol conversion semantics. A schema is enabled only when BRL-CAD has a selector, generated bindings with the correct physical layout, representative inputs or fixtures, and defined conversion policy.

The authoritative source for current modular schemas is ISO’s STEP Module and Resource Library (SMRL) at standards.iso.org/iso/10303/smrl/. Numbered archives such as vN/tech/smrlvN.zip supply published long forms; SMRL v12 is the source used for the AP242 edition 4 binding and includes the published AP203 edition 2 long form. BRL-CAD does not copy the archive wholesale. Incorporated schema files must retain their applicable ISO permission and modification notices.

The historical STEP Module Repository at sourceforge.net/projects/stepmod predates the ISO-hosted SMRL. Its reference area documents early CAx Implementor Forum profiles still encountered in commercial files. ccd_cla_gvp_ast_asd.exp, for example, identifies an interim AP203 extension profile with validation and presentation additions. It is used to verify the AP203e2 compatibility route rather than built as another exporter.

The neighboring ap203_smch_may3_lf.exp is a broader experimental construction-history schema retained as a diagnostic reference. It is not a built-in converter without representative inputs and policies for its added feature and procedural-solid semantics. The repository’s migration discussion records the move to ISO-hosted archives. Historical provenance does not by itself establish redistribution permission; applicable terms must be reviewed before adding a schema to BRL-CAD.

Compatibility names such as CONFIGURATION_CONTROL_3D_DESIGN_MIM_LF, CCD_CLA_GVP_AST, CCD_CLA_GVP_AST_ASD, and AUTOMOTIVE_DESIGN_CC2 are mapped only after their entity definitions and physical layouts are verified. Otherwise the importer uses an edition-isolated binding or reports the schema as unsupported.

AP209 analysis, AP210 electronic assembly, AP224 machining features, and AP238 STEP-NC share integrated resources with the supported mechanical APs, but their domain semantics are not interchangeable. AP232 technical-data packaging and AP239 product-life-cycle support are primarily metadata protocols. Their schemas may inform corpus classification and future plugins; they are not silently routed through AP203, AP214, or AP242.